Basics tutorial
A basic tutorial introduction to gRPC in Go.
Basics tutorial
This tutorial provides a basic Go programmer’s introduction to working with gRPC.
By walking through this example you’ll learn how to:
- Define a service in a
.protofile. - Generate server and client code using the protocol buffer compiler.
- Use the Go gRPC API to write a simple client and server for your service.
It assumes that you have read the Introduction to gRPC and are familiar with protocol buffers. Note that the example in this tutorial uses the proto3 version of the protocol buffers language: you can find out more in the proto3 language guide and the Go generated code guide.
Why use gRPC?
Our example is a simple route mapping application that lets clients get information about features on their route, create a summary of their route, and exchange route information such as traffic updates with the server and other clients.
With gRPC we can define our service once in a .proto file and generate clients
and servers in any of gRPC’s supported languages, which in turn can be run in
environments ranging from servers inside a large data center to your own tablet —
all the complexity of communication between different languages and environments is
handled for you by gRPC. We also get all the advantages of working with protocol
buffers, including efficient serialization, a simple IDL, and easy interface
updating.
Setup
You should have already installed the tools needed to generate client and server interface code – if you haven’t, see the Prerequisites section of Quick start for setup instructions.
Get the example code
The example code is part of the grpc-go repo.
Download the repo as a zip file and unzip it, or clone the repo:
$ git clone -b v1.55.0 --depth 1 https://github.com/grpc/grpc-goChange to the example directory:
$ cd grpc-go/examples/route_guide
Defining the service
Our first step (as you’ll know from the Introduction to gRPC) is to
define the gRPC service and the method request and response types using
protocol buffers.
For the complete .proto file, see
routeguide/route_guide.proto.
To define a service, you specify a named service in your .proto file:
service RouteGuide {
...
}
Then you define rpc methods inside your service definition, specifying their
request and response types. gRPC lets you define four kinds of service method,
all of which are used in the RouteGuide service:
A simple RPC where the client sends a request to the server using the stub and waits for a response to come back, just like a normal function call.
// Obtains the feature at a given position. rpc GetFeature(Point) returns (Feature) {}A server-side streaming RPC where the client sends a request to the server and gets a stream to read a sequence of messages back. The client reads from the returned stream until there are no more messages. As you can see in our example, you specify a server-side streaming method by placing the
streamkeyword before the response type.// Obtains the Features available within the given Rectangle. Results are // streamed rather than returned at once (e.g. in a response message with a // repeated field), as the rectangle may cover a large area and contain a // huge number of features. rpc ListFeatures(Rectangle) returns (stream Feature) {}A client-side streaming RPC where the client writes a sequence of messages and sends them to the server, again using a provided stream. Once the client has finished writing the messages, it waits for the server to read them all and return its response. You specify a client-side streaming method by placing the
streamkeyword before the request type.// Accepts a stream of Points on a route being traversed, returning a // RouteSummary when traversal is completed. rpc RecordRoute(stream Point) returns (RouteSummary) {}A bidirectional streaming RPC where both sides send a sequence of messages using a read-write stream. The two streams operate independently, so clients and servers can read and write in whatever order they like: for example, the server could wait to receive all the client messages before writing its responses, or it could alternately read a message then write a message, or some other combination of reads and writes. The order of messages in each stream is preserved. You specify this type of method by placing the
streamkeyword before both the request and the response.// Accepts a stream of RouteNotes sent while a route is being traversed, // while receiving other RouteNotes (e.g. from other users). rpc RouteChat(stream RouteNote) returns (stream RouteNote) {}
Our .proto file also contains protocol buffer message type definitions for all
the request and response types used in our service methods - for example, here’s
the Point message type:
// Points are represented as latitude-longitude pairs in the E7 representation
// (degrees multiplied by 10**7 and rounded to the nearest integer).
// Latitudes should be in the range +/- 90 degrees and longitude should be in
// the range +/- 180 degrees (inclusive).
message Point {
int32 latitude = 1;
int32 longitude = 2;
}
Generating client and server code
Next we need to generate the gRPC client and server interfaces from our .proto
service definition. We do this using the protocol buffer compiler protoc with
a special gRPC Go plugin. This is similar to what we did in the Quick start.
From the examples/route_guide directory, run the following command:
$ protoc --go_out=. --go_opt=paths=source_relative \
--go-grpc_out=. --go-grpc_opt=paths=source_relative \
routeguide/route_guide.proto
Running this command generates the following files in the routeguide directory:
route_guide.pb.go, which contains all the protocol buffer code to populate, serialize, and retrieve request and response message types.route_guide_grpc.pb.go, which contains the following:- An interface type (or stub) for clients to call with the methods defined in
the
RouteGuideservice. - An interface type for servers to implement, also with the methods defined in
the
RouteGuideservice.
- An interface type (or stub) for clients to call with the methods defined in
the
Creating the server
First let’s look at how we create a RouteGuide server. If you’re only
interested in creating gRPC clients, you can skip this section and go straight
to Creating the client (though you might find it interesting
anyway!).
There are two parts to making our RouteGuide service do its job:
- Implementing the service interface generated from our service definition: doing the actual “work” of our service.
- Running a gRPC server to listen for requests from clients and dispatch them to the right service implementation.
You can find our example RouteGuide server in
server/server.go.
Let’s take a closer look at how it works.
Implementing RouteGuide
As you can see, our server has a routeGuideServer struct type that implements
the generated RouteGuideServer interface:
type routeGuideServer struct {
...
}
...
func (s *routeGuideServer) GetFeature(ctx context.Context, point *pb.Point) (*pb.Feature, error) {
...
}
...
func (s *routeGuideServer) ListFeatures(rect *pb.Rectangle, stream pb.RouteGuide_ListFeaturesServer) error {
...
}
...
func (s *routeGuideServer) RecordRoute(stream pb.RouteGuide_RecordRouteServer) error {
...
}
...
func (s *routeGuideServer) RouteChat(stream pb.RouteGuide_RouteChatServer) error {
...
}
...
Simple RPC
The routeGuideServer implements all our service methods. Let’s look at the
simplest type first, GetFeature, which just gets a Point from the client and
returns the corresponding feature information from its database in a Feature.
func (s *routeGuideServer) GetFeature(ctx context.Context, point *pb.Point) (*pb.Feature, error) {
for _, feature := range s.savedFeatures {
if proto.Equal(feature.Location, point) {
return feature, nil
}
}
// No feature was found, return an unnamed feature
return &pb.Feature{Location: point}, nil
}
The method is passed a context object for the RPC and the client’s Point
protocol buffer request. It returns a Feature protocol buffer object with the
response information and an error. In the method we populate the Feature
with the appropriate information, and then return it along with a nil error
to tell gRPC that we’ve finished dealing with the RPC and that the Feature can
be returned to the client.
Server-side streaming RPC
Now let’s look at one of our streaming RPCs. ListFeatures is a server-side
streaming RPC, so we need to send back multiple Features to our client.
func (s *routeGuideServer) ListFeatures(rect *pb.Rectangle, stream pb.RouteGuide_ListFeaturesServer) error {
for _, feature := range s.savedFeatures {
if inRange(feature.Location, rect) {
if err := stream.Send(feature); err != nil {
return err
}
}
}
return nil
}
As you can see, instead of getting simple request and response objects in our
method parameters, this time we get a request object (the Rectangle in which
our client wants to find Features) and a special
RouteGuide_ListFeaturesServer object to write our responses.
In the method, we populate as many Feature objects as we need to return,
writing them to the RouteGuide_ListFeaturesServer using its Send() method.
Finally, as in our simple RPC, we return a nil error to tell gRPC that we’ve
finished writing responses. Should any error happen in this call, we return a
non-nil error; the gRPC layer will translate it into an appropriate RPC status
to be sent on the wire.
Client-side streaming RPC
Now let’s look at something a little more complicated: the client-side streaming
method RecordRoute, where we get a stream of Points from the client and
return a single RouteSummary with information about their trip. As you can
see, this time the method doesn’t have a request parameter at all. Instead, it
gets a RouteGuide_RecordRouteServer stream, which the server can use to both
read and write messages - it can receive client messages using its Recv()
method and return its single response using its SendAndClose() method.
func (s *routeGuideServer) RecordRoute(stream pb.RouteGuide_RecordRouteServer) error {
var pointCount, featureCount, distance int32
var lastPoint *pb.Point
startTime := time.Now()
for {
point, err := stream.Recv()
if err == io.EOF {
endTime := time.Now()
return stream.SendAndClose(&pb.RouteSummary{
PointCount: pointCount,
FeatureCount: featureCount,
Distance: distance,
ElapsedTime: int32(endTime.Sub(startTime).Seconds()),
})
}
if err != nil {
return err
}
pointCount++
for _, feature := range s.savedFeatures {
if proto.Equal(feature.Location, point) {
featureCount++
}
}
if lastPoint != nil {
distance += calcDistance(lastPoint, point)
}
lastPoint = point
}
}
In the method body we use the RouteGuide_RecordRouteServer’s Recv() method
to repeatedly read in our client’s requests to a request object (in this case a
Point) until there are no more messages: the server needs to check the error
returned from Recv() after each call. If this is nil, the stream is still
good and it can continue reading; if it’s io.EOF the message stream has ended
and the server can return its RouteSummary. If it has any other value, we
return the error “as is” so that it’ll be translated to an RPC status by the
gRPC layer.
Bidirectional streaming RPC
Finally, let’s look at our bidirectional streaming RPC RouteChat().
func (s *routeGuideServer) RouteChat(stream pb.RouteGuide_RouteChatServer) error {
for {
in, err := stream.Recv()
if err == io.EOF {
return nil
}
if err != nil {
return err
}
key := serialize(in.Location)
... // look for notes to be sent to client
for _, note := range s.routeNotes[key] {
if err := stream.Send(note); err != nil {
return err
}
}
}
}
This time we get a RouteGuide_RouteChatServer stream that, as in our
client-side streaming example, can be used to read and write messages. However,
this time we return values via our method’s stream while the client is still
writing messages to their message stream.
The syntax for reading and writing here is very similar to our client-streaming
method, except the server uses the stream’s Send() method rather than
SendAndClose() because it’s writing multiple responses. Although each side
will always get the other’s messages in the order they were written, both the
client and server can read and write in any order — the streams operate
completely independently.
Starting the server
Once we’ve implemented all our methods, we also need to start up a gRPC server
so that clients can actually use our service. The following snippet shows how we
do this for our RouteGuide service:
flag.Parse()
lis, err := net.Listen("tcp", fmt.Sprintf("localhost:%d", *port))
if err != nil {
log.Fatalf("failed to listen: %v", err)
}
var opts []grpc.ServerOption
...
grpcServer := grpc.NewServer(opts...)
pb.RegisterRouteGuideServer(grpcServer, newServer())
grpcServer.Serve(lis)
To build and start a server, we:
- Specify the port we want to use to listen for client requests using:
lis, err := net.Listen(...). - Create an instance of the gRPC server using
grpc.NewServer(...). - Register our service implementation with the gRPC server.
- Call
Serve()on the server with our port details to do a blocking wait until the process is killed orStop()is called.
Creating the client
In this section, we’ll look at creating a Go client for our RouteGuide
service. You can see our complete example client code in
grpc-go/examples/route_guide/client/client.go.
Creating a stub
To call service methods, we first need to create a gRPC channel to communicate
with the server. We create this by passing the server address and port number to
grpc.Dial() as follows:
var opts []grpc.DialOption
...
conn, err := grpc.Dial(*serverAddr, opts...)
if err != nil {
...
}
defer conn.Close()
You can use DialOptions to set the auth credentials (for example, TLS, GCE
credentials, or JWT credentials) in grpc.Dial when a service requires them.
The RouteGuide service doesn’t require any credentials.
Once the gRPC channel is setup, we need a client stub to perform RPCs. We
get it using the NewRouteGuideClient method provided by the pb package
generated from the example .proto file.
client := pb.NewRouteGuideClient(conn)
Calling service methods
Now let’s look at how we call our service methods. Note that in gRPC-Go, RPCs operate in a blocking/synchronous mode, which means that the RPC call waits for the server to respond, and will either return a response or an error.
Simple RPC
Calling the simple RPC GetFeature is nearly as straightforward as calling a
local method.
feature, err := client.GetFeature(context.Background(), &pb.Point{409146138, -746188906})
if err != nil {
...
}
As you can see, we call the method on the stub we got earlier. In our method
parameters we create and populate a request protocol buffer object (in our case
Point). We also pass a context.Context object which lets us change our RPC’s
behavior if necessary, such as time-out/cancel an RPC in flight. If the call
doesn’t return an error, then we can read the response information from the
server from the first return value.
log.Println(feature)
Server-side streaming RPC
Here’s where we call the server-side streaming method ListFeatures, which
returns a stream of geographical Features. If you’ve already read Creating
the server some of this may look very familiar - streaming RPCs are
implemented in a similar way on both sides.
rect := &pb.Rectangle{ ... } // initialize a pb.Rectangle
stream, err := client.ListFeatures(context.Background(), rect)
if err != nil {
...
}
for {
feature, err := stream.Recv()
if err == io.EOF {
break
}
if err != nil {
log.Fatalf("%v.ListFeatures(_) = _, %v", client, err)
}
log.Println(feature)
}
As in the simple RPC, we pass the method a context and a request. However,
instead of getting a response object back, we get back an instance of
RouteGuide_ListFeaturesClient. The client can use the
RouteGuide_ListFeaturesClient stream to read the server’s responses.
We use the RouteGuide_ListFeaturesClient’s Recv() method to repeatedly read
in the server’s responses to a response protocol buffer object (in this case a
Feature) until there are no more messages: the client needs to check the error
err returned from Recv() after each call. If nil, the stream is still good
and it can continue reading; if it’s io.EOF then the message stream has ended;
otherwise there must be an RPC error, which is passed over through err.
Client-side streaming RPC
The client-side streaming method RecordRoute is similar to the server-side
method, except that we only pass the method a context and get a
RouteGuide_RecordRouteClient stream back, which we can use to both write and
read messages.
// Create a random number of random points
r := rand.New(rand.NewSource(time.Now().UnixNano()))
pointCount := int(r.Int31n(100)) + 2 // Traverse at least two points
var points []*pb.Point
for i := 0; i < pointCount; i++ {
points = append(points, randomPoint(r))
}
log.Printf("Traversing %d points.", len(points))
stream, err := client.RecordRoute(context.Background())
if err != nil {
log.Fatalf("%v.RecordRoute(_) = _, %v", client, err)
}
for _, point := range points {
if err := stream.Send(point); err != nil {
log.Fatalf("%v.Send(%v) = %v", stream, point, err)
}
}
reply, err := stream.CloseAndRecv()
if err != nil {
log.Fatalf("%v.CloseAndRecv() got error %v, want %v", stream, err, nil)
}
log.Printf("Route summary: %v", reply)
The RouteGuide_RecordRouteClient has a Send() method that we can use to send
requests to the server. Once we’ve finished writing our client’s requests to the
stream using Send(), we need to call CloseAndRecv() on the stream to let
gRPC know that we’ve finished writing and are expecting to receive a response.
We get our RPC status from the err returned from CloseAndRecv(). If the
status is nil, then the first return value from CloseAndRecv() will be a
valid server response.
Bidirectional streaming RPC
Finally, let’s look at our bidirectional streaming RPC RouteChat(). As in the
case of RecordRoute, we only pass the method a context object and get back a
stream that we can use to both write and read messages. However, this time we
return values via our method’s stream while the server is still writing messages
to their message stream.
stream, err := client.RouteChat(context.Background())
waitc := make(chan struct{})
go func() {
for {
in, err := stream.Recv()
if err == io.EOF {
// read done.
close(waitc)
return
}
if err != nil {
log.Fatalf("Failed to receive a note : %v", err)
}
log.Printf("Got message %s at point(%d, %d)", in.Message, in.Location.Latitude, in.Location.Longitude)
}
}()
for _, note := range notes {
if err := stream.Send(note); err != nil {
log.Fatalf("Failed to send a note: %v", err)
}
}
stream.CloseSend()
<-waitc
The syntax for reading and writing here is very similar to our client-side
streaming method, except we use the stream’s CloseSend() method once we’ve
finished our call. Although each side will always get the other’s messages in
the order they were written, both the client and server can read and write in
any order — the streams operate completely independently.
Try it out!
Execute the following commands from the examples/route_guide directory:
Run the server:
$ go run server/server.goFrom another terminal, run the client:
$ go run client/client.go
You’ll see output like this:
Getting feature for point (409146138, -746188906)
name:"Berkshire Valley Management Area Trail, Jefferson, NJ, USA" location:<latitude:409146138 longitude:-746188906 >
Getting feature for point (0, 0)
location:<>
Looking for features within lo:<latitude:400000000 longitude:-750000000 > hi:<latitude:420000000 longitude:-730000000 >
name:"Patriots Path, Mendham, NJ 07945, USA" location:<latitude:407838351 longitude:-746143763 >
...
name:"3 Hasta Way, Newton, NJ 07860, USA" location:<latitude:410248224 longitude:-747127767 >
Traversing 56 points.
Route summary: point_count:56 distance:497013163
Got message First message at point(0, 1)
Got message Second message at point(0, 2)
Got message Third message at point(0, 3)
Got message First message at point(0, 1)
Got message Fourth message at point(0, 1)
Got message Second message at point(0, 2)
Got message Fifth message at point(0, 2)
Got message Third message at point(0, 3)
Got message Sixth message at point(0, 3)